Stern drive, in particular for twin-propeller boats

ABSTRACT

A stern-drive unit for boats with an inclined hull, wherein the mechanical units are separated from the supporting structure, which is made in the form of a structure ( 10 ) supporting parts ( 7, 5 ) which transmit motion from an inverter ( 6 ) to the propeller, consisting a plate ( 11 ) to be attached to the keel of the boat, to which are connected a pair of V-shaped arms with a first sleeve ( 15 ) fitted at the lower ends thereof, and a third arm with a second sleeve ( 16 ) fitted at the lower end thereof, said sleeves ( 15, 16 ) housing a box ( 17 ) to which lead propeller shaft ( 5 ) and a shaft ( 7 ) orthogonal to it, leading from a motion inverter, seal means being fitted between said box ( 17 ) and said shafts ( 5, 7 ). 
     The drive is light and easily adaptable at low cost to the various keel configurations used by different boat builders.

The invention relates to a stern-drive unit for boats, which is designedto be fitted to boats with an inclined hull.

In the drive according to the invention, the mechanical units areseparated from the supporting structure, which is made in the form of anumber of arms integral with a plate fixed to the keel of the boat; saidarms carry bushings or sleeves that support the mechanical units, whichare covered by a shell designed to enhance the hydrodynamics of thesystem.

The result is a light drive which is easily adaptable at low cost to thevarious keel configurations used by different boat builders.

One of the main problems encountered by boat designers is how to exploitthe small amount of available space to the full, and minimise the weightof the mechanical parts of the boat.

In the field of systems for the transmission of motion to thepropellers, which are one of the parts of the boat in which theseproblems are most acute, an interesting solution, which is the mostadvantageous in terms of size, is the one that uses a “stern drive”.

This is a drive unit wherein the motion output from the engine istransmitted to a substantially vertical shaft which, via a furthertransmission, rotates the propeller shaft, which is generally inclinedby a few degrees from the horizontal plane.

This known configuration provides the greatest space saving, but it isimpractical, because a motion inverter unit has to be associated withthe transmission device; the overall cost is very high, with the resultthat this system is uncompetitive, and there is no demand for it.

This problem was solved by the present applicant with a drive unitforming the subject of patent application PC 2002 A 027, which comprises(i) a transmission with two coaxial bevel gears, fitted opposite oneanother on the same engine shaft, which engage a bevel gear mounted on ashaft orthogonal to the preceding one, and (ii) means designed to makeone or other of said bevel gears integral with said engine shaft, inorder to control the rotation of the shaft leading to the propellers inone direction or the opposite direction.

However, the considerable advantages offered by this solution arelimited by the need to adapt the supporting structure of the mechanicalparts in this type of drive to the keel configuration, which varies fromone boat builder to another.

In order to understand the problem, reference should be made to theschematic drawing in FIG. 1, which shows a boat keel in cross-sectionalong a plane orthogonal to the boat axis, and FIG. 2, which shows astern drive of known type.

In the figures, no. 1 indicates the keel of a boat with a V-shaped hulland slanting walls. No. 2 indicates the stiffening structures of theboat, which are arranged longitudinally, with engine unit 3 mounted onthem. Engine output shaft 4 is connected to propeller shaft 5 via adrive which, in known solutions, comprises a motion inverter 6 and avertical shaft 7, which in turn is connected to propeller shaft 5 via abevel gear pair or other known system.

In these known drives, the parts downstream of the inverter are housedin a casing 8 made of metal of suitable thickness (approx. 20-30 mm),integral with a plate 9 used to secure the whole assembly to the boat.

Said casing has a considerable weight, of several hundred kilos, whichit would be useful to reduce as far as possible.

This structure, as well as being very heavy, is very expensive if asingle specimen or a small production run is made, because the cost ofthe moulds used to make the casing will be spread between a very smallnumber of parts.

FIG. 1 shows two possible embodiments of the casing.

In the casing shown on the left-hand side of the figure, plate 10 isinclined in relation to the body which encloses shaft 7, at an angleequal to the angle α formed between the bottom wall of the boat and thehorizontal plane.

The axis of shaft 7 intersects with the axis of engine shaft 4, which isthe ideal solution in kinematic terms, because it enables motion to betransmitted between the two shafts via a simple bevel gear pair.However, that solution requires a different casing to be made for eachtype of boat, to adapt the assembly to the specific keel configuration.

Conversely, the configuration of the casing shown on the right-hand sideof the same figure is easily applicable to boats with differentlyinclined hulls, because plate 9 is substantially orthogonal to the bodythat contains shaft 7. However, as the axis of shaft 7 does notintersect with engine shaft 4, but passes at a distance “d” from it,depending on the geometry of the assembly, further parts are required totransmit motion between the two shafts, and the torque createddischarges considerable forces onto the structure, which must beabsorbed by suitably strengthening the structure.

These problems are now solved by the present invention, which relates toa stern-drive unit wherein the mechanical parts are separated from thesupporting structure, which can thus be easily made with a configurationadaptable to the type of boat in question without excessively onerousmodifications.

Basically, in the unit according to the invention, the casing consistsof a plate to which are fixed two V-shaped arms which terminate with asleeve, and a third straight central arm, which also terminates with asleeve, aligned with the preceding one.

The mechanical parts are made in the form of separate units, which areassembled and mounted on said sleeves, and the whole assembly iscompleted by a fairing made of fibreglass or other light, low-costmaterial.

The fact that the structure is made in the form of a plate to whichthree arms are fixed means that modifications are very easily made, sothat the structure can be adapted to different boat configurations withlittle work, and no need for complex, expensive operations.

The invention will now be described in detail, by way of example but notof limitation, by reference to the annexed figures wherein:

FIGS. 1 and 2 schematically illustrate a cross-section of a boat and aview of a drive unit made according to the prior art;

FIG. 3 is a schematic side view of a drive unit according to theinvention;

FIGS. 4 and 5 are perspective views, from below, of a drive unitaccording to the invention;

FIG. 6 is an exploded perspective view of the unit.

As shown in FIGS. 4 and 6, the drive unit according to the inventioncomprises a supporting structure 10, on which are mounted the variousparts that transmit motion from inverter 6 to the propeller.

In greater detail, said supporting structure comprises a plate 11designed to be fixed to the structure of the boat in accordance with aknown technology, to which three arms, 12, 13 and 14, are welded orotherwise secured.

The first two of said arms, 12 and 13, are located in the front part ofthe plate and arranged in a V shape; the upper ends are at a distancefrom one another and welded to the plate, while the lower ends convergeand are welded to a bushing or sleeve 15, coaxial with propeller shaft5.

The third shaft 14 is located in the rear part of plate 11, in a centralposition, and supports a second sleeve 16, coaxial with the precedingone, at the lower end.

Propeller shaft 5 and vertical shaft 7, which is orthogonal to it, leadto a box 17, which is supported by sleeves 15 and 16.

Shaft 7 passes into a cylindrical body or box 18, and the two shafts 5and 7 are connected via an angular transmission of known type notillustrated in the figure.

Seal elements, also of known type, are inserted between shafts 5 and 7and their respective boxes.

The structure is preferably, but not necessarily, completed by afairing, made of a light material such as fibreglass, composite orplastic, consisting of one or more parts 19, which is designed toimprove the hydrodynamics of the system.

The solution described, which requires the mechanical parts to beseparated from the supporting structure, solves the problems of weightand easy adaptability to various boat designs.

The boxes, with systems that provide the seal around the rotatingshafts, are known and can be made at limited cost, while the supportingstructure described above, with a pair of sleeves fixed to the ends ofarms welded to a plate secured to the boat, is light and easily adaptedto a wide variety of hull inclinations, merely by varying the length ofthe arms, with no need to modify moulds or anything else.

The drive unit structure obtained with the solution described is lightand easily adapted to a wide variety of keel geometries at low cost.

1) A drive unit for stern-drive boats, which includes a structure (10)that supports the parts (7, 5) which transmit motion from an inverter(6), located at engine output (3), to the propeller, consisting of aplate (11) secured to the keel of the boat, to which are attached aplurality of arms (12, 13, 14) which support means (15, 16) designed tohouse box-shaped elements (17, 18) into which said drive parts pass. 2)A drive unit for stern-drive boats as claimed in claim 1, wherein saidstructure (10) which supports the drive parts consists of a plate (11)to which are connected a pair of V-shaped arms, with a first sleeve (15)fitted at the lower ends thereof, and a third arm with a second sleeve(16) fitted at the lower end thereof, which said sleeves (15, 16) housea box (17) to which lead propeller shaft (5) and a shaft (7) orthogonalto it, leading from a motion inverter, seal means being fitted betweensaid box (17) and said shafts (5, 7); 3) A drive unit for stern-driveboats as claimed in claim 2, wherein said V-shaped arms (12, 13) arelocated in the front part of the plate, and said third arm (14) islocated at the rear, in a central position. 4) A drive unit forstern-drive boats as claimed in claim 3, wherein said sleeves (15, 16)are aligned. 5) A drive unit for stern-drive boats as claimed in claim4, which includes a shell designed to enclose said box (17), which saidshell consists of elements (19) designed to be fitted to said sleeves(15, 16) to produce a casing designed to improve the hydrodynamics ofthe system. 6) A drive unit for stern-drive boats as claimed in claim 1,which includes: a plate (11) attached to the keel of the boat, to whichare connected a pair of V-shaped arms (12, 13) with a first sleeve (15)fitted at the lower ends thereof: a third arm (14) with a second sleeve(16) fitted at the lower end thereof; a box (17) mounted on said sleeves(15, 16), to which said box lead propeller shaft (5) and a shaft (7)orthogonal to it, which leads from a motion inverter; seal means betweensaid box (17) and said shafts (5, 7); 7) A drive unit for stern-driveboats as claimed in claim 6, wherein a motion inverter unit (6) isfitted between engine shaft (4) and said drive shaft (7), which saidmotion inverter unit comprises a transmission with two coaxial bevelgears, fitted opposite one another on the same engine shaft, whichengage a conical cogwheel mounted on said drive shaft (7), and meansdesigned to make one or other of said bevel gears integral with saidengine shaft. 8) A drive unit for stern-drive boats as claimed in claim7, which includes a shell designed to enclose said box (17), which saidshell consists of elements (19) designed to be fitted to said sleeves(15, 16) to produce a casing designed to improve the hydrodynamics ofthe system.